1. Field of the Invention
The present invention relates to an optical head that focuses a beam irradiated from the light source in an information recording surface of an optical disc through a transparent base plate on the recording surface to record and reproduce an information signal on and from the recording surface.
2. Description of the Prior Art
Generally, in an optical disc system optical head that focuses a beam irradiated from the light source in an information recording surface of an optical disc through a transparent base plate on the recording surface, an information signal is recorded on or reproduced from the recording surface. The spot size of the beam on the recording surface is preferably small enough to obtain a good property for recording and reproducing.
The more homogeneous is the strength of the beam being incident on an objective lens, the smaller the spot size is tightened. However, the light irradiated from a semiconductor laser used as a laser source for the optical head generally has a Gaussian distribution intensity. Therefore, the truncation of the objective lens is increased to render the optical intensity at effective radius of the objective lens to be close to the center intensity, so that the beam intensity distribution can become homogeneous, as shown in FIG. 1. In FIG. 1, the horizontal line represents rim intensity, i.e., the ratio of the optical intensity at effective radius of the objective lens to the center optical intensity, and the vertical line represents the spot size, which is 1 when the rim intensity is 0 (zero). Referring to FIG. 1, the higher the rim intensity is, that is, the larger the truncation of the objective lens becomes, the smaller the spot size is tightened. The rim intensity depends on the size of the incident beam against the effective radius of the objective lens. In this optical system wherein the beam from the light source is collimated into parallel rays by a collimator lens and the parallel rays are subsequently incident on the objective lens, the size of incident beam is proportional to a focal length of the collimator lens. Therefore, the focal length will be determined so as to obtain the desired rim intensity with the objective lens.
The cross section of the spot can not become a true circle, where the rim intensity changes in a circumferential direction, since the rim intensity and the spot size have such a relationship as shown in FIG. 1. The Intensity distribution of the irradiation ray in a horizontal direction parallel to the junction face of the semiconductor laser differs from the intensity distribution of the irradiation ray in a vertical direction orthogonal thereto, so far as the Gaussian distribution of the irradiation ray is concerned. If the angle of full width at half maximum (hereinafter called as F.W.H.M.) in the horizontal direction and that in the vertical direction is expressed by xcex8h, and xcex8v, respectively, the ratio xcex8h/xcex8v is generally within the range from 1/2 to 1/3 and, therefore, the cross sectional shape of the beam becomes a long ellipse in the vertical direction. When the elliptic beam is converged by the objective lens, the rim intensity in the horizontal direction becomes lower than that in the vertical direction, and the shape of a beam spot on the surface of the optical disc becomes an ellipse, which has a spot size in the horizontal direction that is larger than in the vertical direction. If it is necessary to rectify the elliptic beam spot to become a beam spot of a true circle, an optical beam shaping system that closes the beam size in the horizontal direction with the beam size in the vertical direction is used. For example, two prisms may be used as the optical beam shaping system as shown in FIG. 2. Referring to FIG. 2, a beam 12 is irradiated from a semiconductor laser 11, and is then collimated to parallel rays by a collimator lens 13 before the collimated beam 12 is successively transmitted through a prism 14 and a prism 15. The prism 14 magnifies the beam size from D1 to D2 on a plane parallel to the surface of the paper, but does not magnify the beam on the plane perpendicular to the surface of the paper. Notes that the prism 15 magnifies the beam size from D2 to D3 as shown in FIG. 2, however, the magnification ratio D3/D2 of the prism 15 is as well as the ratio D2/D1 of the prism 14. Therefore, adjusting the horizontal direction of the junction plane of the semiconductor laser 11 to become parallel to the surface of the paper results in beam shaping. The magnification may be determined in order to obtain a desired spot shape.
As above mentioned, it is preferable to shape and extend the focal length of the collimator lens for the optical head. However, the collimator lens acts to increase the amount of the beam fluxes vignetted to thereby decrease the efficiency of utilization of the beam. Considering the rim intensity and the efficiency of utilization of the beam, a feasibly balanced combination of the focal length of the collimator lens 13 and the magnification of the prism should be selected. This effect will be explained with reference to FIG. 3. In FIG. 3, the horizontal line represents the magnification of beam shaping and the vertical line represents the focal length fCL of the collimator lens. In the example shown in FIG. 3, the angle of F.W.H.M. in the horizontal direction and that in the vertical direction are xcex8h=11xc2x0and xcex8v=27xc2x0, respectively. The effective radius of the objective lens is 3.4 mm. Referring to FIG. 3, the curve A is in the case of the 35% rim intensity in the horizontal direction, the curve B is in the case of the 40% rim intensity in the vertical direction, and the curve C is in the case of the efficiency xcex7=45% of utilization of the beam.
According to the above principle, the above curve A and curve B, and below curve C, is indicated by hatching in FIG. 3. If a combination of the focal length fCL and the magnification M of beam shaping is selected from the area, then the combination can satisfy the condition of the 35% rim intensity in the horizontal direction, the 40% rim intensity in the vertical direction, and the 45% efficiency of utilization of the beam. When the magnification M of beam shaping is lower than 2.5, the cross sectional shape of the collimated rays cannot become a true circle. Referring to FIG. 1, however, because the spot size will change a small amount at a rim intensity larger than 20% or 30%, the optical head having a good performance can be provided.
By the way, in the optical beam shaping system as shown in FIG. 2, an astigmatism will be caused, when the beam being incident on prism 14 and 15 is not parallel rays. When the semiconductor laser 11 is displaced from the focal point of the collimator lens 13, the beam may not be parallel rays, so that the astigmatism will be caused. FIG. 4 shows a simulation of relationship between a displacement of the laser source and the astigmatism. The semiconductor laser source irradiates a laser having a wavelength 650 nm, and an angle of F.W.H.M. xcex8h=11xc2x0 in the horizontal direction and xcex8v=27xc2x0 in the vertical direction respectively. The magnification M of beam shaping is 2.5, and the focal length fCL=8.0 mm. Referring to FIG. 4, the astigmatism is proportional to the displacement of the laser source, and the proportional coefficient is 5.7 mxcex/xcexcm. If the astigmatism is 30 mxcex, in order to have no influence against recording and reproducing the signal, the maximum permissible displacement will be about 5.3 pm.
An interval between the laser source and the collimator lens may change with temperature. Assuming that a base stand supporting the optical member is made of an aluminum alloy, and a temperature range xcex94T in which the performance of an optical head is warranted is xcex94T=xc2x130 degrees, the displacement xcex94Z of the interval can be estimated as follows.
xcex94Z=fCLxc2x7xcex94Txc2x7xcex1=5.3 xcexcm
Notes that xcex1 is an expansion coefficient of the aluminum alloywhich is 2.3xc3x9710xe2x88x925 degreexe2x88x921. The displacement xcex94Z may be equal to the maximum permissible displacement, and therefore we consider the astigmatism caused by temperature shift may be permitted.
In this case of the wavelength of about 650 nm, the good spot size and the good efficiency of utilization of the beam can be compatible and the astigmatism of the optical beam shaping system can be in the permissible range.
Recently, the short wavelength laser source having a wavelength not longer than 500 nm is developed to realize the optical disc having a high-density recording. When the short wavelength laser source is used in the optical head system, the wave front aberration is inversely proportional to the wavelength, then the astigmatism will be made larger than at a wavelength of about 650 nm. For example, when a semiconductor violet laser having a wavelength 400 nm is used, the astigmatism will be about 1.6 times as large as at wavelength of 650 nm. Therefore, the maximum permissible displacement xcex94Z of interval between the source and the collimator lens will be 0.6 times lower than at a wavelength of 650 nm. When beam shaping is performed on the same condition, the astigmatism caused by the displacement of the interval between the source and the collimator lens, and then particularly, the recording and reproducing performance will be remarkably dropped due to the temperature shift.
Japanese Patent No. 2,933,325 shows the beam shaping system, which adjusts a location of the collimator lens by positively making use of the displacement of the interval between the source and the collimator lens, so that the astigmatism can be compensated for. However, this system can be applied to only stable astigmatism, the variable astigmatism due to temperature shift can not be cancelled by this system. If a drive system of the collimator lens in the optical axis will be provided, then temperature shift may be compensated, but this will result not only in increase of the number of members, but also the necessity of both a servo circuit for controlling the location of the lens and a studying program for adjusting the lens.
Therefore, it is an object of the present invention to provide an optical head using a laser having a wavelength not longer than 500 nm, which maintains the recording and reproducing performance, and good temperature property.
In accordance with one aspect of the present invention, there is provided an optical head including a laser source, a collimator lens, an optical beam shaping system, and an objective lens. The laser source irradiates the laser beam having a wavelength not longer than 500 nm. The collimator lens collimates the laser beam to a beam having parallel rays. Additionally, the optical beam shaping system adjusts the cross sectional shape of the beam from the collimator lens. The objective lens converges the beam on an optical information medium. Then, the optical head has a relationship between a focal length fCL of the collimator lens and a magnification M of beam shaping that satisfies the following equation:       A    SO    ≥                    (                  C                      f            CL                          )            2        ⁢    arc    ⁢          xe2x80x83        ⁢          sin      ⁡              [                                            (                              M                -                1                            )                                      (                                                                    n                    2                                    ⁢                  M                                -                1                            )                                      ]            
Note that C is a constant in (xcexxc2x7mm)1/2, ASO is a maximum astigmatism per displacement of the collimator lens from the focal position, and n is a refractive index of a member of optical beam shaping system.
The constant C may be no smaller than 29 and no greater than 33 in (xcexxc2x7mm)1/2.
The optical beam shaping system may include a prism. Preferably, the system may include two prisms.
The relationship between the focal length fCL and the magnification M may satisfy the following three equations A, B, and C:                               M          ·                      f            CL                          ≥                                            R              OL                                      sin              ⁢                                                θ                  h                                2                                              ⁢                                    -                                                ln                  ⁢                                      xe2x80x83                                    ⁢                  2                                                  ln                  ⁡                                      (                                          I                                              rim                        ·                        h                                                              )                                                                                                          (        A        )            
Note that M is the magnification of beam shaping prism, fCL is the focal length of the collimator lens, ROL is an effective radius of the objective lens, xcex8h is the angle of F.W.H.M. in the horizontal direction, and Irimxc2x7h is the rim intensity of the beam in the horizontal direction.                               f          CL                ≥                                            R              OL                                      sin              ⁢                                                θ                  v                                2                                              ⁢                                    -                                                ln                  ⁢                                      xe2x80x83                                    ⁢                  2                                                  ln                  ⁡                                      (                                          I                                              rim                        ·                        v                                                              )                                                                                                          (        B        )            
Note that xcex8v is the angle of F.W.H.M. in the vertical direction, and Irimxc2x7v is the rim intensity of the beam in the vertical direction.                     η        ≥                              2                                          π                ·                                  R                  h                                            ⁢                              R                v                                              ⁢                      ∫                                          ∫                S                            ⁢                                                exp                  ⁡                                      [                                                                  -                        2                                            ⁢                                              {                                                                                                            (                                                              x                                                                  R                                  h                                                                                            )                                                        2                                                    +                                                                                    (                                                              y                                                                  R                                  v                                                                                            )                                                        2                                                                          }                                                              ]                                                  ⁢                                  ⅆ                  x                                ⁢                                  ⅆ                  y                                                                                        (        C        )            
Note that xcex7 is an efficiency of utilization of the beam, and Rh and Rv are 11e2 effective radius of the Gaussian distribution in the horizontal direction and in the vertical direction, respectively, and is indicated as following equations:             R      h        =                            2                      ln            ⁢                          xe2x80x83                        ⁢            2                              ⁢              M        ·                  f          CL                    ⁢      sin      ⁢                        θ          h                2                        R      v        =                            2                      ln            ⁢                          xe2x80x83                        ⁢            2                              ⁢              f        CL            ⁢      sin      ⁢                        θ          v                2            
Preferably, the rim intensity of the beam in the horizontal direction may be 0.35, the rim intensity of the beam in the vertical direction may be 0.40, and the efficiency of utilization of the beam may be 0.45.
In a further aspect of the present invention, there is provided an optical disc apparatus including a laser source, a collimator lens, an optical beam shaping system, an objective lens, an optical detector, and a signal processor. The laser source irradiates the laser beam having a wavelength not longer than 500 nm, and the collimator lens collimates the laser beam to the parallel rays. Then, the optical beam shaping system adjusts the cross sectional shape of the beam from the collimator lens. Additionally, the objective lens converges the beam on an optical information medium. The optical detector outputs current due to an optical beam reflected by the surface of the information medium. The signal processor processes the current from the optical detector and picks up the desired signal from the signal. Then the optical disc apparatus has a relationship between a focal length fCL of the collimator lens and a magnification M of beam shaping that satisfies the following equation:       A    SO    ≥                    (                  C                      f            CL                          )            2        ⁢    arc    ⁢          xe2x80x83        ⁢          sin      ⁡              [                                            (                              M                -                1                            )                                      (                                                                    n                    2                                    ⁢                  M                                -                1                            )                                      ]            
Note that C is a constant in (xcexxc2x7mm)1/2, ASO is a maximum astigmatism per displacement of the collimator lens from the focal position, and n is a refractive index of a member of optical beam shaping system.
According to the optical head of the present invention, except for the rim intensity condition and the efficiency of utilization condition, the new equation limiting the astigmatism caused in the optical beam shaping system is used to select a combination of the magnification of beam shaping and the focal length of the collimator lens. Therefore, the optical head having the good performance in recording and reproducing at a wavelength not longer than 500 nm, at which the astigmatism should be restricted, can be provided.